1. Field of the Invention
The present invention relates to a printing apparatus and a method of printing. More specifically, the present invention pertains to a printing apparatus that drives a print head to form a raster line, which includes an array of dots arranged in one direction of a printing medium, and carries out a sub-scan, which feeds the printing medium relative to the print head in another direction that crosses the raster line whenever the raster line is formed, thereby printing an image, as well as to a method of such printing.
In this specification, the raster line that includes an array of dots arranged in one direction of the printing medium implies an array of dots created by at least one of the dot-forming elements without a feed in the sub-scanning direction. The direction of the dot array is hereinafter referred to as the main scanning direction, and the direction crossing the dot array as the sub-scanning direction.
2. Discussion of the Background
The printing apparatus, especially an ink jet printer, causes dot-forming elements, such as nozzles, mounted on a print head to spray ink against a printing medium and create dots of the ink on the surface of the printing medium, so as to implement printing. One typical arrangement of the ink jet printer has a print head that scans the surface of the printing medium (sheet of printing paper) to form raster lines.
A known ink jet printer of this arrangement has a nozzle array on the print head, which includes a plurality of nozzles arranged at a predetermined pitch in the sub-scanning direction. The print head of this structure simultaneously prints a plurality of lines by the plurality of nozzles in one main scan (pass).
In the case of the ink jet printer with such a print head, some scatter in properties of the respective nozzles or scatter in pitch between the plurality of nozzles causes the occurrence of banding and thereby deteriorates the picture quality of an image printed on the printing medium.
One known printing technique to prevent such deterioration of the picture quality is constant pitch sub-scan printing. This printing technique is also called interlace printing. The constant pitch sub-scan printing uses a nozzle array on a print head, which includes a plurality of nozzles arranged in the sub-scanning direction at intervals of an integral multiple of a dot pitch that corresponds to the dot of the printing resolution.
The printing medium is fed in the sub-scanning direction by a constant distance corresponding to n dot pitch after each pass, where N nozzles (where N is a positive integer) are arranged in the sub-scanning direction in the nozzle array, n nozzles (where n is a positive integer of not greater than N) are actually driven among the N nozzles arranged in the nozzle array, and the pitch between the nozzles is equal to k dot pitch, which is the minimum pitch of dots created on the printing medium (where k is a positive integer not greater than n and is prime to n).
In the constant pitch sub-scan printing technique, adjoining raster lines in the sub-scanning direction are printed by different nozzles. Even when there is some scatter in properties and pitch of the respective nozzles, this arrangement effectively reduces the deterioration of the picture quality of the printing image due to the scatter and thereby ensures printing of the high picture quality.
In addition to printing of letters and characters, the recent trend requires printing of multi-tone images, such as photographic images, at high quality. A variety of improvements have been given to the ink jet printer to enable printing of fine dots and meet the requirement. A known technique to print a multi-tone image doubles the driving frequency of the ink jet nozzles in the main scanning direction and thereby enhances the dot density in the main scanning direction. A proposed technique to enhance the dot density in the sub-scanning direction decreases the feeding amount of the printing medium in the sub-scanning direction to ensure finer sheet feeding.
Even when the constant pitch sub-scan printing technique is applied to attain high-quality printing, accumulation of errors of sheet feeding (feeding errors) may result in banding. By way of example, in constant pitch sub-scan printing, it is assumed that the upper dot out of two consecutive dots in the sub-scanning direction is printed by a last nozzle included in a selected group of nozzles in a specific pass, whereas the lower dot is printed by a first nozzle included in the selected group of nozzles. In this case, the passes for creating these dots are discontinuous in time. This causes a large accumulated error of sheet feeding between the two dots and facilitates the occurrence of banding.
In the recent ink jet printers, the constant pitch sub-scan printing is implemented by adopting an overlap printing technique that causes dots printed in a subsequent pass to partly overlap the dots printed in a preceding pass, with a view to realizing high-quality printing. If there is a significant time interval between printing of a dot in the preceding pass and the printing of a dot in the subsequent pass, the ink of the dot in the preceding pass is dried up before the dot is printed in the subsequent pass. This results in a poor combination of these two dots and causes a significant difference in printing density, compared with other overlapped portions. This undesirably causes the occurrence of banding.
The above description of the problem of the deteriorating picture quality is in reference to an ink jet printer that creates dots, while the print head moves in the main scanning direction. This problem is, however, not restricted to an ink jet printer, but may be found in any printers that create an image as a set of dots, such as a thermal wax-transfer printer. The problem also arises in the printing apparatuses that carry out the feed in the sub-scanning direction, whereas the print head is not required to move in the main scanning direction.
The object of the present invention is thus to provide a printing apparatus with a plurality of dot-forming elements arranged in the sub-scanning direction, which solves the problem of the prior art printing apparatuses that create an image as a set of dots and carries out the high-quality printing without deterioration of the picture quality due to banding.
At least part of the above and the other related objects is realized by a printing apparatus that drives a print head to form a raster line, which includes an array of dots arranged in one direction in a printing medium, and carries out a sub-scan, which feeds the printing medium relative to the print head in another direction that crosses the raster line whenever the raster line is formed, thereby printing an image. The printing apparatus includes:
a dot-forming element array that is mounted on the print head and includes N dot-forming elements for creating dots on the printing medium, the N dot-forming elements being arranged at a k dot pitch, which corresponds to a minimum pitch between dots created on the printing medium, in the sub-scanning direction;
a print head driving unit that drives the print head and causes required dot-forming elements in the dot-forming element array to create dots on the printing medium;
a sub-scan control unit that carries out the feed of the printing medium in the sub-scanning direction after each pass of raster creation, which drives the print head driving unit to form at least part of the raster line; and
a dot creation control unit that selects n dot-forming elements (where n is a positive integer of less than N) among the dot-forming element array and causes a raster line to be formed on the printing medium in each pass, the dot creation control unit causing a remaining dot-forming element other than the n dot-forming elements selected among the dot-forming element array create a dot adjoining to two dots, which are created by two passes having a significant time interval between them.
The present invention is also directed to a method corresponding to this printing apparatus. This method drives a print head to form a raster line, which includes an array of dots arranged in one direction of a printing medium, and carries out a sub-scan, which feeds the printing medium relative to the print head in another direction that crosses the raster line whenever the raster line is formed, thereby printing an image. The method includes the steps of:
driving the print head and causing a dot-forming element array mounted on the print head to create dots on the printing medium, wherein the dot-forming element array includes N dot-forming elements for creating dots on the printing medium and the N dot-forming elements are arranged at a k dot pitch, which corresponds to a minimum pitch between dots created on the printing medium, in the sub-scanning direction;
carrying out the feed of the printing medium in the sub-scanning direction after each pass of raster creation to form at least part of the raster line; and
selecting n dot-forming elements (where n is a positive integer of less than N) among the dot-forming element array and causing a raster line to be formed on the printing medium in each pass, while causing a remaining dot-forming element other than the n dot-forming elements selected among the dot-forming element array to create a dot adjoining two dots, which are created by two passes having a significant time interval between them.
In the printing apparatus and the corresponding method of the present invention, the dots are created on the printing medium by the dot-forming element array, which includes a plurality of dot-forming elements arranged at a predetermined pitch in the sub-scanning direction. The structure of the present invention carries out the feed of the printing medium in the sub-scanning direction after each pass, which drives the print head driving unit and forms at least part of a raster line. The procedure carries out the control to select n dot-forming elements among the dot-forming element array and form a raster line on the printing medium by a certain pass. The procedure also causes a dot adjoining the two dots created by the two passes having a significant time interval between them to be created by a dot-forming element other than the selected dot-forming elements among the dot-forming element array. This arrangement of the invention utilizes the dot-forming element that is not selected in the conventional dot creating process, and thereby enhances the utilization ratio of the dot-forming elements. No new pass is required to create a dot adjoining the two dots created by the two passes having a significant time interval between them. This structure does not undesirably extend the time required for printing.
The two passes having a significant time interval between them may be two passes that are discontinuous in time. The discontinuous passes in time facilitate accumulation of the feeding errors in the sub-scanning direction. It is accordingly effective to cause the unselected dot-forming element to create a dot that adjoins to the two dots created by the two discontinuous passes in time.
There are a variety of printing techniques that create close dots by the two passes having a significant time interval between them. By way of example, when s dot-forming elements included in the n dot-forming elements aligned in the sub-scanning direction in the dot-forming element array are used to form one raster line, a value n/s and the pitch k of the dot-forming elements are prime to each other, and the printing medium is fed by n/s dot pitch in the sub-scanning direction after each pass. This arrangement enables the selected n dot-forming elements to efficiently create dots by the interlace technique.
In the printing apparatus of such a structure, one possible application drives the sub-scan control unit and the print head driving unit and causes part of the selected dot-forming elements to print dots in a subsequent pass, which partly overlap dots created in a preceding pass by the selected dot-forming elements. This arrangement causes one raster line to be formed by a plurality of dot-forming elements. This effectively cancels the problem due to a scatter of the dot-forming elements.
The two dots formed by the two passes having the significant time interval between them may adjoin each other in the sub-scanning direction or may adjoin each other in both the sub-scanning direction and the main scanning direction.
The n dot-forming elements selected among the N dot-forming elements may not include end dot-forming elements of the dot-forming element array. In this case, the dot adjoining the two dots created by the two passes having the significant time interval between them is formed by each dot-forming element of the dot-forming element array. Banding often occurs in the end of the dot-forming element array. Formation of the dot adjoining the two dots created by the two passes by the end dot-forming element thus effectively prevents the occurrence of banding.
When the interval between the plurality of dot-forming elements is not less than 2 dot pitch, the use of all the N dot-forming elements may cause an interlace condition to fail. Such failure of the interlace condition occurs, for example, in the case where the number of effective dot-forming elements N/s, which is determined by taking into account the number of dot-forming elements s used to create a raster line, is not an integer or in the case where the number of effective dot-forming elements N/s is not prime to the dot pitch k between the dot-forming elements, while the number of effective dot-forming element N/s is equal to a feeding amount L in the sub-scanning direction. In these case, n dot-forming elements (where n is an integer of less than N) are selected among the N dot-forming elements. Such selection enables the n dot-forming elements to satisfy the interlace conditions. Each of unselected (N-n) dot-forming elements is used to create a dot adjoining the two dots created by the two passes having a significant time interval. This arrangement implements the interlace printing with no special pass, thereby ensuring the advantages of dot creation without extending the printing time.
In accordance with another preferable application, a different number of dot-forming elements are selected among the dot-forming element array for part of a plurality of passes from a number of dot-forming elements selected for the other passes. The feed of the printing medium in the sub-scanning direction is then carried out according to the number of selected dot-forming elements. The number of dot-forming elements used for each pass may not be fixed at a constant value. The feed in the sub-scanning direction depends upon the number of selected dot-forming elements. It is not necessary to fix the number of dot-forming elements, each of which is used to create a dot adjoining the two dots created by discontinuous passes. All or part of the unselected dot-forming elements may be applied for such dot-forming elements.
A typical example of the dot-forming element in the printing apparatus is a nozzle that spouts ink and creates dots on the printing medium. The principle of the present invention is, however, also applicable to other printing apparatuses in which ink is not spouted, for example, an impact dot matrix printer and a thermal wax-transfer printer. The respective units or steps of the present invention may be actualized electrically by memories and controllers. The controller may be a general-purpose control element, such as a CPU, or an exclusive control circuit.
The printing apparatus of the present invention having any one of the above structures may be the type that forms raster lines through main scans that reciprocate the head relative to the printing medium as well as the type that forms raster lines without such main scans.
In any of the printing apparatuses of the present invention discussed above, the computer may control the head for recording dots and the sub-scan according to a preset program. Another application of the present invention is a recording medium, in which such a program is recorded.
The present invention is thus directed to a recording medium, in which a program is recorded in a computer readable manner, wherein the program causes the computer to drive a print head to form a raster line, which includes an array of dots arranged in one direction of a printing medium, and carry out a sub-scan, which feeds the printing medium relative to the print head in another direction that crosses the raster line whenever the raster line is formed, thereby printing an image. The program realizes the functions of:
driving the print head and causing a dot-forming element array mounted on the print head to create dots on the printing medium, wherein the dot-forming element array includes N dot-forming elements for creating dots on the printing medium and the N dot-forming elements are arranged at a k dot pitch, which corresponds to a minimum pitch between dots created on the printing medium, in the sub-scanning direction;
carrying out the feed of the printing medium in the sub-scanning direction after each pass of raster creation to form at least part of the raster line; and
selecting n dot-forming elements (where n is a positive integer of less than N) among the dot-forming element array and causing a raster line to be formed on the printing medium in each pass, while causing a remaining dot-forming element other than the n dot-forming elements selected among the dot-forming element array to create a dot adjoining -to- two dots, which are created by two passes having a significant time interval.
The computer executes the program recorded in the recording medium to actualize the printing apparatus of the present invention discussed above. Available examples of the recording media include flexible disks, CD-ROMs, magneto-optic discs, IC cards, ROM cartridges, punched cards, prints with barcodes or other codes printed thereon, internal storage devices (memories like a RAM and a ROM) and external storage devices of the computer, and a variety of other computer readable media. Another application of the present invention is a program supply apparatus that supplies a computer program, which causes the computer to realize the control functions of the printing apparatus, via a communications path.